Add Two backgrounds option

[jmborr]

Short description of the changes:

Include an option for the reflectivity data to use one or two background ranges

Long description of the changes:

Check list for the pull request

• I have read the [CONTRIBUTING]
• I have read the [CODE_OF_CONDUCT]
• I have added tests for my changes
• I have updated the documentation accordingly

Check list for the reviewer

• I have read the [CONTRIBUTING]
• I have verified the proposed changes
• best software practices
  • all internal functions have an underbar, as is python standard
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

Manual test for the reviewer

References

[jmborr]

@mdoucet I'm going to fix the couple of tests that are still failing, then I'll let you know with another request for your review/approval.